When an examinee moves during an imaging operation in an MRI apparatus for obtaining a tomogram of an examination site of the examinee by using the NMR phenomenon, the effect extends to the overall image, and such an artifact that an image streams in a phase encode direction (hereinafter referred to as “body movement artifact”) occurs. This is because when an echo signal at each lattice point on a measurement space (K space) is sampled, the sampling which is parallel to a frequency encode direction is repeated in a phase encode direction. Such a measuring method will be hereinafter referred to as an orthogonal system (Cartesian) sampling method.
In addition to the orthogonal system (Cartesian) sampling method, a non-orthogonal system (Non-Cartesian) sampling method is known. There are representatively known a radial sampling method (see non-patent document 1, for example), and a hybrid radial method or propeller MRI method which corresponds to the combination of the radial sampling method with a phase encode (see non-patent document 2, for example).
The radial sampling method is a technique of radially scanning a measurement space to sample data while the rotation angle is varied with substantially one point (generally, origin) of the measurement space set as a rotation center, thereby obtaining echo signals required to reconstruct one image. When imaging is performed by using the radial sampling method, a body movement artifact is scattered around an image because sampling is radially executed, and thus the artifact concerned gets out of a visual field to be noted. Therefore, as compared with the imaging based on orthogonal system sampling method, the body movement artifact is inconspicuous, and it called as being robust to the body movement.
In the radial sampling method, the distribution of a reading gradient magnetic field in an imaging plane is different among echo signals because the measurement space is radially scanned. Accordingly, the effects of non-uniformity of a magnetostatic field distribution and non-linearity of the gradient magnetic field are different among echo signals. Furthermore, an applied amount of the gradient magnetic field which is pre-calculated without any regard for non-linearity of the gradient magnetic field and offset is different from an actually applied amount of the gradient magnetic field, and thus echo signals cannot be arranged at accurate coordinates of the measurement space. From these circumstances, the radial sampling method has a problem that an artifact caused by non-linearity of the gradient magnetic field or the like is more liable to occur in an image as compared with the orthogonal system sampling method.
There is known a method of measuring the non-linearity of the gradient magnetic field in advance before execution of a sequence and reflecting it to an actual measurement in order to correct the artifact in the radial sampling method (see non-patent document 3, for example).
Non-patent document 1: G. H. Glover et. al., Projection Reconstruction Techniques for Reduction of Motion Effects in MRI, Magnetic Resonance in Medicine 28: 275-289 (1992)
Non-patent document 2: James G. Pipe, Motion Correction with PROPELLER MRI: Application to Head Motion and Free-Breathing Cardiac Imaging, Magnetic Resonance in Medicine 42: 963-969 (1999)
Non-patent document 3: D. C. Peters et. al., Centering the Projection Reconstruction Trajectory: Reducing Gradient Delay Errors, Magnetic Resonance in Medicine 50: 1-6 (2003)